Steel for high-strength components made of bands, sheets or tubes having excellent formability and particular suitability for high-temperature coating processes

ABSTRACT

A steel for high-strength components including bands, sheets or pipes having excellent formability and particular suitability for high-temperature coating processes above Ac 3  (about 900° C.) is disclosed. The steel includes the following elements (contents in % by mass): C 0.07 to ≦0.15, Al≦0.05, Si≦0.80, Mn 1.60 to ≦2.10, P≦0.020, S≦0.010, Cr 0.50 to ≦1.0, Mo 0.10 to ≦0.30, Ti min  48/14×[N], V 0.03 to ≦0.12, B 0.0015 to ≦0.0050, with the balance iron including usual steel-accompanying elements.

The invention relates to steel for high-strength components made ofbands, sheets or pipes having excellent formability and particularsuitability for high-temperature coating processes according to claim 1.The term high temperature in this context indicates temperatures aboveA_(c3) (about 900° C.).

Modern lightweight construction of components made of steel intended tohave the greatest possible resource utilization by way of maximum weightsavings increasingly requires the use of high-strength steels.

This applies, for example, to the tinplate or sanitary industry, theconstruction of chemical equipment, the power plant technology and moreparticularly the automobile industry with the goal to reduce the fleetfuel consumption.

Components made of high-strength steels employed in the automobileindustry are typically coated with corrosion-inhibiting coatings,predominantly made of zinc. In other of the aforementioned applicationfields, enamel coatings are also used in addition tocorrosion-inhibiting coatings.

Semi-finished goods, such as bands or sheets made of conventionalhigh-strength steels for these application fields are predominantlyproduced by thermo-mechanical rolling. This requires that the steels arenot subjected to additional heat treatment in subsequent processingsteps, because the mechanical properties obtained with thethermo-mechanical treatment would otherwise be lost.

If the steels are subjected to a subsequent thermal treatment where, forexample, a corrosion-inhibiting layer in form of enamel or metalliccoatings made of zinc, aluminum or their alloys is applied at treatmenttemperatures reaching the values higher than A_(c3) (about 900° C.),then these steels loose their original strength. This situation occurslikewise also in similarly heat-treated zones after welding.

This phenomenon is repeated if multiple heat treatments are preformed,for example with thermal coating methods, with intersecting weld seamsin the respective heat-treated region, as well during repeated enamelfirings typically performed during enameling, causing the material tocontinuously loose strength.

The following Table 1 shows this phenomenon on the example of the steelgrade S-420 in 3.0 mm and 8.0 mm, respectively, with a minimum yieldstrength of 420 MPa.

TABLE 1 Change in the mechanical properties of sheets made of S-420after 1 and 2 enamel firings, respectively. Thickness, Sample R_(p0.2) -mm orientation State MPa R_(m) - MPa A₈₀ - % 3.0 mm Longitudinal Initialstate 444 569 21.3 after 1 anneal 430 521 25.1 after 2 anneals 405 52225.9 Transverse Initial state 502 592 20.9 after 1 anneal 453 537 25.1after 2 anneals 439 537 23.0 8.0 mm Longitudinal Initial state 426 52329.8 after 1 anneal 391 498 31.0 after 2 anneals 385 494 31.5 TransverseInitial state 437 538 26.7 after 1 anneal 401 505 29.7 after 2 anneals395 503 29.2

This loss in strength after corresponding heat treatment is even morepronounced in high-strength multiphase steels, because the originalmartensitic phase fraction disappears during heating above thetransition temperature A_(c3), if the cooldown is not controlled andintensified.

Another problem which may occur with high-strength steels is asignificant increase in the solubility for hydrogen during heating aboveA_(c3). The hydrogen then remains in the material structure duringaccelerated cooldown which may cause formation of cracks in thematerial.

For this reason, steels are in demand which produce a hard structurealso during slow cooldown (for example, in still air).

These steels may have another problem in that egression of hydrogen fromthe material is hindered by a thick protective layer, such as enamel. Ifthis is the case, then the coating may be at risk of spalling (fishscales).

Fish scales indicate defects in the enamel, which no longer guarantee acontinuous protection of the steel substrate. A high resistance of theenamel component against fish scales is therefore important whenenameling steel.

It is generally assumed that the occurrence of fish scales is caused bycontact of the steel surface with humidity from the furnace atmosphereand from the enamel slurry during the enameling process.

The reaction of water with the steel surface causes formation of atomichydrogen which diffuses into the steel during the firing process.

After firing of the enamel at about 900° C. and subsequent cooldown, thesolubility of hydrogen in steel decreases, and the hydrogen is drivenout of the steel and recombines at the material boundary steel/enamel toform molecular hydrogen.

This reaction is accompanied by an increase in volume, wherein locally ahigh pressure can be generated which finally becomes so large that theyield strength of the composite enamel/steel is exceeded andhalf-moon-shaped enamel splinters (fish scales) occur on the enamel thathas meanwhile solidified.

For cold-rolled or hot-rolled sheets, a number of conventional steelsare known which are resistant to fish scale formation. Because of thefrequently required particular deep-drawing properties, these steels aretypically designed as low-strength IF steels (e.g., EP 0 386 758 B1) andare based on alloy concepts where the fish scale resistance is producedby cementite precipitates broken down by cold-rolling at the grainboundaries, with the atomic hydrogen accumulating at the cementiteprecipitates and hence rendered harmless with respect to fish scaleformation.

High-strength, readily formable steels suitable for high-temperaturetreatments, for example during the enameling, are not known up till now.The requirements for a high-strength steel, which need not always besatisfied at the same time, can be summarized for the aforedescribedapplication fields as follows:

-   -   High material strength of the component after forming also after        heat treatment at temperatures above 900° C.,    -   Fish scale resistance after enameling.    -   Good formability,    -   Generally good weldability,    -   Good high-frequency induction (HFI) and laser weldability in the        production of pipes,    -   Suitable for zinc-plating of the component.

The invention is based on the invention to produce a low-cost steel forhigh-strength components made of bands, sheets or pipes, which hasexcellent formability and suitablility for high-temperature coatingmethods, while simultaneously ensuring general weldability and, moreparticularly, HFI weldability.

According to the teaching of the invention, this object is obtained witha steel having the following composition in % by mass:

C 0.07 to ≦0.15 Al ≦0.05 Si ≦0.80 Mn 1.60 to ≦2.10 P ≦0.020 S ≦0.010 Cr0.50 to ≦1.0 Mo 0.10 to ≦0.30 Ti_(min) 48/14 × [N] V 0.03 to ≦0.12 B0.0015 to ≦0.0050with balance iron, including usual steel-accompanying elements.

The high-strength steel according to the invention is designed asheat-treated steel which can be hardened in air or in a medium withcomparable cooldown gradients. The steel is particularly suited forhigh-temperature coating methods, for example in enameling orzinc-plating, even at treatment temperatures above 900° C., and isdistinguished in that it does not lose strength during cooldown aftercoating, but even becomes stronger as a result of the heat treatment. Itwas surprising to persons skilled in the art to observe in extensivetest series, that for the first time a steel could be provided with thealloy composition of the invention, which has both an excellentenameling ability and fish scale resistance, while attaining at the sametime a high strength as a result of the heat treatment during enamelfiring or during zinc-plating.

This comparatively very cost-effective alloying concept, in particularthe low carbon content, also produces excellent cold-forming propertiesin the initial state “soft”, which is of particular importance for usewith deep-drawn parts, for example in sanitary installations for hotwater heaters, in boiler construction, in the construction of chemicalequipment or in the construction of automobile chassis.

The relatively low carbon equivalent furthermore ensures excellentgeneral weldability. Weldability is excellent, in particular, withhigh-frequency induction welding (HFI welding), as used for example inthe production of pipes, because the chromium content in the weld seam,which prevents unwanted chromium carbide precipitates, is relativelysmall.

The fish scale resistance of the steel is attained with the inventionthrough addition of chromium and vanadium, wherein finely dispersedprecipitates of chromium and vanadium carbides or carbon nitrites andtitanium nitrites form hydrogen traps in the hard structure of thesteel, with the atomic hydrogen formed during enameling accumulating atthe hydrogen traps without damaging the enamel.

The alloy concept based on Mn, Cr, Mo, V and B enables temper-hardeningof the steel already with a cooldown gradient that is comparable tocooldown in air through an advantageous shift in the relevanttransformation points.

This presumes that, according to the present invention, the existingnitrogen in the steel is completely bound in form of titanium nitritesthrough addition of titanium, in order to prevent boron nitriteprecipitates and to thereby ensure the effectiveness of the added boron.Accordingly, according to the invention, at least a stoichiometricaddition of titanium relative to the nitrogen content must bemaintained.

According to an advantageous embodiment of the invention, the steel hasa low Si-content of ≦0.30% for zinc-plating, thereby ensuringsuitability for zinc-plating, for example, for applications in theautomotive industry.

Conventional temper-hardened steels are known where hardening isattained solely by cooldown of the steel in air, for example after heattreatment of the component, in order to realize the required materialproperties.

If the steel cools down after hot-rolling at least partially in air sofast that the air hardening effect sets in, then cold-formability can beattained by way of a subsequent soft-annealing process, for example in ahood-type annealing furnace, or by homogenizing annealing.Alternatively, the cold-formability after hot-rolling can also bemaintained by slowly cooling a suitably tightly wound coil, optionallyin a special insulated hood.

After cold-forming or shaping, the temper-hardening conditions can thenagain be adjusted by way of a subsequent heat treatment.

The term cold-forming refers to the following process variants:

-   a) The direct production of corresponding components from hot-band    by deep drawing and the like with subsequent optional heat    treatment.-   b) Further processing into pipes using suitable drawing and    annealing processes. The pipes themselves are subsequently made into    components, for example by bending, internal high-pressure forming    (IHU) and the like, and subsequently temper-hardened.-   c) Further processing of the hot-band into cold-band with subsequent    annealing and shaping process. The cold-band is subsequently    processed by deep-drawing and the like, as described under a) or b).

The following Table 2 lists parameters measured on samples of the steelaccording to the invention for hot-rolled and cold-rolled sheets orbands, as well as pipes produced therefrom:

TABLE 2 Change in the mechanical parameters of the steel of theinvention after enameling. R_(p0.2) [MPa] R_(m) [MPa] A₅ [%] Cold band1.5 mm Delivery state soft 339 494 35.1 After enameling 490 770 12.1 Hotband 4.6 mm Delivery state soft 336 528 33.4 After enameling 475 74012.2

The pickling removal and the fish scale resistance of sheets made of thesteel of the invention as well as of three comparative steels withhigher strengths were tested with respect to their suitability forenameling.

The test results for the suitability of the steel of the invention forenameling in comparison to other higher-strengths types of steels aresummarized in the following Table 3. The tests for pickling removal andfish scale resistance of the sheets were performed according to thestandard EN 10209.

For testing the fish scale resistance, a boiler test enamel was used inaddition to the cold-band test frit Ferro 2290.

TABLE 3 Comparison of the enameling results Steel Comparison ComparisonComparison according to steel steel steel the invention H420LAD MS1200TRIP HXT800 Sheet metal Sheet metal Sheet metal Sheet metal Test Nominalvalue thickness 1.5 mm thickness 2.5 mm thickness 1.5 mm thickness 1.0mm Pickling 20-50 g/m² 45 25 49 212 removal Fish scale test No result Noresult More than 30 More than 30 Test not Boiler test (see FIG. 1a) (seeFIG. 1b) (see FIG. 1c) possible enamel Fish scale test No result Noresult More than 30 More than 30 Test not Ferro RTU possible 2290

The test results show that the comparison steel TRIP HXT800 has apickling removal which is significantly higher than the allowed value,so that fish scale resistance could not be tested.

The pickling removal for the two comparison steels was within the limitsfor the target values; however, there was no fish scale resistance.

The results of the fish scale test are illustrated in FIGS. 1 a to 1 c.

The change of the mechanical parameters of the steel of the inventionduring enameling compared to other higher-strength steels is illustratedin the following Figures. No values after enameling could be determinedfor the comparison steel HXT800, because the steel cannot be enameleddue to excessive pickling removal.

Reference is made here to FIGS. 2 and 3.

The advantages of the steel according to the invention can be summarizedas follows:

-   -   High material strength also after heat treatment above 900° C.,    -   Fish scale resistance after enameling of the component,    -   Significantly increased strength on the finished component and        therefore possibility for lightweight construction by reducing        the thickness compared to conventional enameled steels,    -   Very good weldability of the steel, in particular also with HFI        welding in the context of pipe production,    -   Excellent cold formability of the steel in non-temper-hardened        state and therefore possibility for producing complex        components,    -   The steel can be zinc-plated,    -   Suitability for non-metallic protective layers.

The following typical parameters for hot-rolled or cold-rolled sheetsand pipes in a soft-annealed state are listed below for the steel of theinvention:

R_(el) and/or R_(p0.2) 310-430 [MPa] R_(m) 450-570 [MPa] A₅ ≧23 [%]

In the heat-treated state, for example after enameling or galvanizingabove 900° C., the following exemplary mechanical parameters areattained:

R_(el) and/or R_(p0.2) 450-600 [MPa] R_(m) 700-850 [MPa] A₅ ≧12 [%]

The steel according to the invention can be used in many applications inform of a band, sheet, hot- or cold-rolled, or for welded and seamlesspipes.

For cold-rolled or cold-formed products, the thickness range orwall-thickness range may be, for example, 0.5-4 mm. The correspondingvalues for hot-rolled or hot-formed products are about 1.5-8 mm.

1.-13. (canceled)
 14. A steel for high-strength components of bands,sheets or pipes, comprising by mass percent: C 0.07 to ≦0.15 Al ≦0.05 Si≦0.80 Mn 1.60 to ≦2.10 P ≦0.020 S ≦0.010 Cr 0.50 to ≦1.0 Mo 0.10 to≦0.30 Ti_(min) 48/14 × [N] V 0.03 to ≦0.12 B 0.0015 to ≦0.0050,

with balance iron, including usual steel-accompanying elements.
 15. Thesteel of claim 14, having a C-content of 0.08 to ≦0.10%.
 16. The steelof claim 14, having a Si-content of ≦0.30%.
 17. The steel of claim 14,having a Mn-content of 1.80 to ≦2.0%.
 18. The steel of claim 14, havinga Cr-content of 0.70 to ≦0.80%.
 19. The steel of claim 14, having aMo-content of 0.15 to ≦0.25%.
 20. The steel of claim 14, having aTi-content of 0.02 to ≦0.03%.
 21. The steel of claim 14, having aV-content of 0.05 to ≦0.10%.
 22. The steel of claim 14, having aB-content of 0.0025 to ≦0.0035%.
 23. The steel of claim 14 forapplication in a high-temperature coating process above Ac₃ (about 900°C.).
 24. A component formed from a readily formable band, sheet or tubemade of a steel comprising, in mass percent: C 0.07 to ≦0.15 Al ≦0.05 Si≦0.80 Mn 1.60 to ≦2.10 P ≦0.020 S ≦0.010 Cr 0.50 to ≦1.0 Mo 0.10 to≦0.30 Ti_(min) 48/14 × [N] V 0.03 to ≦0.12 B 0.0015 to ≦0.0050,

with balance iron, including usual steel-accompanying elements, whereinthe component is, after forming, heat-treated at a temperature above Ac₃(about 900° C.) and has after cooldown a minimum yield strength of 450MPa.
 25. The component of claim 24, wherein the heat treatment includesenameling with one or more firings.
 26. The component of claim 24,wherein the heat treatment includes a metallic coating.
 27. Thecomponent of claim 26, wherein the coating is a zinc-plating.
 28. Amethod of making a component, comprising the steps of: forming astructure selected from the group consisting of band, sheet, and tubeinto a shape; heat-treating the structure at a temperature above Ac₃(about 900° C.); and allowing the structure to cool down to produce acomponent with a minimum yield strength of 450 MPa.